In what appears to a recurring paradox of the Libyan conflict, heartening news of advances by Libyan rebel forces aided by renewed NATO bombing strikes is being off-set by wider warnings from Western officials about the operation becoming an open-ended slog. As a result, even as reports from the ground now depict setbacks to loyalists of leader Muammar Gaddafi, concern continues rising about the ultimate success of the intervention in Libya—and whether its outcome may not to a large degree determine the very future of NATO.

On Wednesday NATO leaders planned to meet with UK officials in London to discuss the Libyan mission in the wake of a reinvigorated bombing campaign that included strikes on Gaddafi targets in Tripoli Tuesday night. Reports Wednesday said that resurgence of air attacks had allowed modest advances by rebel troops in past days, notably in their efforts to take the fiercely defended town of Zintan southwest of the capital. But despite those developments, comments from Western capitals this week have heightened worry that the wider Libyan operation has already bogged down into what will wind up being a long, horizon-less conflict that will seriously test political resolve and military finances. And that, some fear, will pose real questions about the relevance of NATO-led interventions in post-Cold War clashes.On Monday, British Navy commander Admiral Mark Stanhope said that while the intervention NATO took charge of March 31 was in no danger of stalling as it approached the end of its second 90-day mandate, viability questions would arise following what now appears to be the mission’s inevitable extension. “Certainly in terms of NATO’s current time limit that has been extended to 90 days, we are comfortable with that,” Stanhope told reporters in comments widely interpreted as indicating more money and military assets would have to be provided by participating countries over time to keep the operation from flagging further. “Beyond that, we might have to request the government to make some challenging decisions about priorities.”

Read “challenging decisions about priorities” as a British euphemism for “making costly, unpopular choices about major escalation—or termination”. And Stanhope’s remarks aren’t the only ones airing concern about the Libyan intervention going on and on. On Tuesday, even as the Canadian parliament voted massively to extend its activities in the NATO-led operation, opposition leaders warned they’d reverse their support if the conflict continued on indefinitely without signs of Gaddafi’s ouster.

Perhaps more significantly, U.S. Republican House Speaker John Boehner publicly warned President Barack Obama Tuesday that extension of American military involvement in Libya beyond the approaching end of the initial three-month period without a vote from Congress would violate the War Powers Resolution law. Boehner said if Obama prolonged that U.S. participation when the 90-day limit ends June 19, lawmakers from both parties would file litigation against him. That came following Monday’s debate of GOP presidential hopefuls who uniformly dumped on Obama’s decision to deploy U.S. forces in the Libyan operation.

Though Republican rivals staking contrary positions to those of an Obama they hope to unseat is par for the course, they aren’t alone in their particularly sour view on the Libyan intervention. Polls show the Libyan mission to be increasingly unpopular with a majority of American voters. In addition to battle fatigue from previous operations in Iraq and Afghanistan, American unhappiness over involvement in Libya is also linked to its minimum $608 million cost. That outlay abroad comes as political leaders plan to further slash spending on state programs–even as the economy shows signs of new stalling, and increased pain of lower incomes and unemployment. Quite simply, many Americans feel limited funds could be better spent elsewhere than over Libya.

And they aren’t the only ones allowing their opinion on Libya to be shaped by the pinch on their pocketbooks. France’s initial $720,000 daily contribution to the effort has now ballooned to over $2 million per day. Reports now indicate even more spending may be required to help already over-extended French military assets to keep pace with a prolonged conflict. Estimates from the UK suggest its price tag for the Libyan intervention could rise to nearly $1.5 billion through September—an expense political opponents would jump on to fuel growing public opposition to further spending cuts under the Conservative government’s massive austerity program.

But if the current debt and deficit reduction priorities on both sides of the Atlantic will limit just how much capitals are willing to contribute to the Libyan operation over time, earlier willingness—and stinginess—in facilitating NATO operations had already begun raising serious questions about the alliance’s very future. Just last Friday, outgoing U.S. Defense Secretary Robert Gates delivered withering criticism of Europe’s limited readiness to pull its weight in the organization. Gates warned of a “dim future” if NATO continues being divided between members “willing and able to pay the price and bear the burdens of alliance commitments, and those who enjoy the benefits of NATO membership…but don’t want to share the risks and the costs.”

In terms of the Libyan conflict, that was interpreted as Gates challenging the UK and France—countries that led the push for intervention—to step up, take control, and see the mission to success as the U.S. focuses its efforts on other operations. But even more importantly, the warning seemed to issue a particularly stark challenge by Washington to its continental allies: either start contributing in even bigger ways to NATO than you ever have before—economic slowdowns and austerity be damned—are be prepared to watch from its margins as the alliance becomes orphaned by the U.S. largely going its own way.